1. The Field of the Invention
The invention relates to a process for accurately measuring particles which are deposited on a surface. More specifically, the invention relates to a process for accurately measuring the diameter of particles which are used to calibrate industrial particle scanning tools.
2. Technical Background
Particle scanners are used in the chip manufacturing industry to detect contamination on extremely smooth surfaces such as silicon wafer surfaces. Superior quality and performance in the silicon wafer surfaces is achieved if the surfaces are free of imperfections and contaminations. Particle scanners operate by sensing scattered light from imperfections and contaminations on the surface as a laser is scanned over the surface.
The particle scanners are calibrated before scanning wafer surfaces to ensure their sensitivity and accuracy. Calibration of the particle scanners involves the particle scanners measuring the light scatter off of particles of predesignated size. In this manner, the accuracy of the particle scanners readings is compared with the known particle size.
One common type of particle deposited on the wafer surface are spherical polystyrene latex spheres (PSL's). In most cases the diameter of these spheres is known only to about 10%. However, it has been shown by National Institute of Science and Technology (NIST), that sphere diameters vary by as much as 25% depending on sphere size and sphere manufacturer. Because the scatter varies significantly with sphere diameter, knowing the true diameter of a PSL is an important scanner calibration issue. Greater calibration accuracy is achieved by more accurately knowing the PSL diameter.
PSL diameters are extremely small and may range from micrometers to nanometers. Often times improving the measurement of PSL's to less than 10% error is not economically feasible using conventional methods.
Various technologies exist for measuring PSL diameters and include using scattered light to take measurements. Many of these methods make use of particles suspended in a fluid (either gas or liquid).
A technique currently employed in the semiconductor industry is to measure the size of the particles in an electrostatic classifier. Charged particles, moving in a flow of air, are turned towards a small opening through the influence of an electric field generated by an applied voltage. Resistance to turning comes almost entirely from air resistance, which depends on particle diameter. Thus the amount of bending is determined by a combination of particle diameter, flow rate and applied electric field. Only particles of a particular size (regardless of material composition) get through the opening at a fixed voltage. The determination of particle diameter is made from a first principles calculation of this process and relies on having very accurate values of system geometry, applied voltage and flow rate. Accurate measurements from this method often have a 10% or more error rate.
Industry practice has had to accept inaccurately measured PSL's for calibration testing. An important issue for developing particle scanners is optimizing sensitivity for a production environment. Improved calibration methods would greatly enhance particle scanner sensitivity.
Thus, it would be an advancement in the art to produce a process to accurately, reliably, and economically measure PSL sphere diameter.
Such a method is disclosed and claimed herein.